It is known that the carboxylic acid amide can be produced by synthesizing an ammonium carboxylate salt from a carboxylic acid and ammonia and subjecting the salt to a dehydration reaction.
The carboxylic acid amide can be produced by a batch process or a continuous process. In an industrial process, conditions for separating and purifying the carboxylic acid amide as a product and conditions for removing and recycling by-products and unreacted raw materials are greatly influenced by, for example, the types and properties of desired carboxylic acid amide, individual starting materials and by-products. In particular, the cost and availability of raw materials, the final yield of desired product after separation and purification, the simplicity of entire production steps including a separation/purification system, the cost of production facility construction, the operation efficiency, the utility cost, etc. are extremely important factors, and an industrial process must be decided by collectively judging all of these. When the carboxylic acid amide is produced by, for example, a batch process, conventionally, the reaction temperature is set at 150 to 220.degree. C. and the reaction time at 12 to 30 hr. and water formed as a by-product by the reaction is removed from the top of the reactor as a low boiling point fraction.
When the carboxylic acid amide is produced by this process, the conversion of the ammonium carboxylate can be 90% or higher. However, by-products such as nitrile compounds and carboxylic acid amide dimerization products are formed in relatively large amounts, so that the selectivity of carboxylic acid amide is as low as 90 to 95% and the final yield of carboxylic acid amide is also not satisfactory. Moreover, the production of carboxylic acid amide by the batch reaction prolongs the processing time and increases the formation of by-products, so that the above process cannot be regarded as an industrially advantageous process from a judgement of its productivity and economy.
For example, U.K. Patent No. 935,391 and Japanese Patent Laid-open Publication No. 57(1982)-38754 disclose a continuous process for producing a carboxylic acid amide, in which a carboxylic acid or an ammonium carboxylate is continuously fed into a reaction column through its top or its middle while ammonia is continuously fed into the reaction column through its lower part to thereby obtain a carboxylic acid amide.
In this process, with respect to reaction starting materials, ammonia is used in excess of the carboxylic acid. The excess ammonia is distilled out from the top of the reaction column together with water. For condensing the fraction from the top of the reaction column to thereby attain a recovery, a cooling medium of extremely low temperature must be used. Further, it is needed to install a separator for distilling water off recovered aqueous ammonia and install an absorption column for recovering ammonia gas obtained by this separation. Therefore, the above process cannot be regarded as a process which is satisfactory from the viewpoint of economy and operation efficiency.
In the process described in these published specifications, much of ammonia gas formed by a a decomposition reaction of ammonium carboxylate is distilled outside the system from the top of the reaction column. Therefore, the above process involves problems such that not only is the yield of carboxylic acid amide relative to the amount of ammonia added as a starting material unsatisfactory but also by-products such as nitrile compounds and carboxylic acid amide dimerization products (dimerized amide of carboxylic acid) are formed in relatively large amounts.
Therefore, there is a demand for the development of a process for producing a carboxylic acid amide, which enables producing a highly purified carboxylic acid amide at a high yield from a carboxylic acid and ammonia as raw materials.
In addition to the above processes, a process for producing a carboxylic acid amide has been proposed, in which the carboxylic acid amide is produced from an ammonium carboxylate in the presence of a dehydration catalyst. In this process, molybdenum oxide, an alkyltin catalyst, a mixture of silica gel and alumina or a titanium tetrachloride catalyst is used as the dehydration catalyst.
In this process, the dehydration reaction can be carried out at relatively low temperatures, so that not only can the formation of by-products such as nitrile compounds and carboxylic acid amide dimerization products be suppressed but also the reaction time can be shortened. However, when the carboxylic acid amide is produced with the use of the catalyst, it is apprehended that the catalyst be dissolved or otherwise contained in the obtained carboxylic acid amide and, hence, a step of separating or otherwise removing the catalyst from the carboxylic acid amide after the completion of the reactions becomes requisite. If costs of catalyst production and catalyst removal incurred by the use of the catalyst, etc. are taken into account, this process also cannot be regarded as a satisfactory process.
The applicant of the present patent application proposed in Japanese Patent Laid-open Publication No. 9(1997)-157233 a process for producing a carboxylic acid amide represented by the formula: RCONH.sub.2 (wherein R represents a saturated alkyl group having 1 to 4 carbon atoms), comprising:
a first step comprising supplying, to a reactor consisting of a rectifying tower and a reaction vessel, a carboxylic acid represented by the formula RCOOH (wherein R is as defined above) and an ammonium salt thereof (provided that, in the total supply, the molar amount of the carboxylic acid is in the range of from 0.1 to 2.0 times that of the ammonium salt), which are fresh or recycled from the following second and third steps; conducting a dehydration reaction of the ammonium salt at 130 to 200.degree. C.; and distilling mixture components consisting of formed water and the carboxylic acid and/or unreacted ammonium salt off the top of the rectifying tower to thereby obtain components containing the carboxylic acid amide from the reaction vessel; PA1 a second step comprising distilling the components containing carboxylic acid amide obtained in the first step to thereby purify the carboxylic acid amide; and PA1 a third step comprising distilling water off the mixture components consisting of water and carboxylic acid and/or unreacted ammonium salt recovered in the first step. PA1 wherein the dehydration reaction of the saturated aliphatic ammonium carboxylate is conducted while supplying water to a reaction system in which the dehydration reaction is being carried out. PA1 [I] a first step comprising: PA1 feeding to a reaction vessel, as raw materials, PA1 (a) the saturated aliphatic carboxylic acid, ammonia and water, and/or PA1 (b) the saturated aliphatic ammonium carboxylate and water, PA1 and conducting the dehydration reaction of the saturated aliphatic ammonium carboxylate in the presence of water to thereby form the saturated aliphatic carboxylic acid amide and water; PA1 [II] a second step comprising distilling (preferably under reduced pressure) a reaction mixture obtained in the first step containing the saturated aliphatic carboxylic acid amide and water, in a first rectifying tower so that the reaction mixture is separated into low boiling point components containing the saturated aliphatic ammonium carboxylate and water and high boiling point components containing the saturated aliphatic carboxylic acid amide, thereby obtaining the saturated aliphatic carboxylic acid amide; and PA1 [III] a third step comprising distilling off part of water from the low boiling point components obtained in the second step in a second rectifying tower to thereby obtain an aqueous solution of saturated aliphatic ammonium carboxylate and feeding the aqueous solution to the first step.
This process enables producing a highly purified saturated lower alkyl carboxylic acid amide simply at a high yield with an industrial advantage, as compared with the above prior art processes. However, this process has room for improvement because by-products such as dicarboxylic acid amides and carboxylic acid salt of amidine are formed in a considerable amount in the dehydration reaction of the ammonium carboxylate for producing the carboxylic acid amide.
On the other hand, the obtained reaction mixtures containing carboxylic acid amide and water, including that obtained by the carboxylic acid amide producing process described in the above literature, generally also contain unreacted raw material ammonium carboxylate, etc. The reaction mixtures can be fractionated into low boiling point components such as ammonium carboxylate and water and high boiling point components such as carboxylic acid amide by a purification according to the customary distillation method. Thus, a carboxylic acid amide of relatively high purity can be obtained.
However, the lower aliphatic carboxylic acid amide has a relatively high boiling point and its thermal stability is poor at a temperature zone near the boiling point, so that, for example, the formation of impurities (by-products) is likely to occur in the purification stage. Main incidentally formed impurities include dicarboxylic acid amides which are amides resulting from dimerization of the carboxylic acid. The dicarboxylic acid amide and the carboxylic acid amide have approximately identical boiling points, so that these are obtained as high boiling point components. It is extremely difficult to purify the carboxylic acid amide by separating the same from the dicarboxylic acid amide with the use of customary distillation and purification means.
Further, it is known that this dicarboxylic acid amide is decomposed into a carboxylic acid and a nitrile compound when exposed to high temperature.
When it is intended to produce, for example, an N-vinyl carboxylic acid amide (one of useful vinyl monomers) by reacting a starting material of a carboxylic acid amide containing the above dicarboxylic acid amide as impurity with acetylene or the like at high temperatures, a decomposition reaction of the dicarboxylic acid amide simultaneously occurs in the production stage with the result that a carboxylic acid and a nitrile compound are formed. The formed carboxylic acid causes a deterioration of the reaction mixture containing the N-vinyl carboxylic acid amide, thereby inducing, for example, formation of polymers. As a result, troubles occur such as clogging of lines for production of the N-vinyl carboxylic acid amide, and the yield of the N-vinyl carboxylic acid amide drops. Therefore, it is not desirable to directly use the carboxylic acid amide containing the dicarboxylic acid amide as a starting material in the synthesis of the N-vinyl carboxylic acid amide.
The purification method combining the distillation method and the recrystallization method is generally known as providing means for obtaining a highly purified carboxylic acid amide not containing any dicarboxylic acid amide, which is suitably used as a starting material in the synthesis of, for example, an N-vinyl carboxylic acid amide. The use of the recrystallization method in combination with the distillation method provides An effective means for removing the dicarboxylic acid amide whose separation is difficult only by the distillation method. However, this recrystallization method generally employs an aqueous solution of alcohol in which the solubility of carboxylic acid amide is relatively high as a recrystallization solvent, so that, after the recrystallization, the carboxylic acid amide is present at a relatively high concentration in the mother liquor. Further, in this method, the yield of the carboxylic acid amide is so low that the mother liquor must be reprocessed for enhancing the yield. Still further, this method requires a drying step for removing the recrystallization solvent, which is disadvantageous from the viewpoint of cost.
For using the carboxylic acid amide containing dicarboxylic acid amide as a starting material in the synthesis of, for example, an N-vinyl carboxylic acid amide, it is required that some pretreatment be carried out to convert the dicarboxylic acid amide to components which are not detrimental to the production of an N-vinyl carboxylic acid amide, etc.
As mentioned above, the reaction mixture containing the carboxylic acid amide and water can be fractionated into high boiling point components composed mainly of the carboxylic acid amide and low boiling point components composed mainly of an ammonium carboxylate and water by the distillation method.
If recovered, the above ammonium carboxylate can be reutilized as a raw material for producing the carboxylic acid amide, and itself is useful as, for example, a dye buffer, an organic synthesis buffer, a raw material for drug or a raw material for organic synthesis.
It is important to remove water from the low boiling point components containing the ammonium carboxylate and water to thereby recover and reutilize the ammonium carboxylate from the viewpoint that the quantity of waste is reduced and that the carboxylic acid amide is produced at a high yield.
The removal of water from the low boiling point components (mixture) containing the ammonium carboxylate and water can be performed by the recrystallization method. The water separation according to the recrystallization method is performed by either cooling to below the crystallization temperature of the mixture or adding an to the ammonium carboxylate. In these recrystallization organic solvent such as an alcohol which is a poor solvent methods, the solubility of ammonium carboxylate in water is so high that the ammonium carboxylate is left in relatively high concentration in the residual liquid remaining after the recovery of ammonium carboxylate. For obtaining the ammonium carboxylate at a high yield, it is needed to further recover the ammonium carboxylate from the residual liquid. In the cooling crystallization method, when the water concentration (water content) is high, measures must be taken such that the crystallization temperature is set at an extremely low or that an organic solvent is added. When an organic solvent is used, it is additionally needed to implement a drying step for removing the organic solvent adhering to obtained crystals of ammonium carboxylate and a step for recycling the organic solvent. From the viewpoint of facility cost, productivity, etc. of these steps, the purification according to the cooling crystallization of ammonium carboxylate with the use of the organic solvent cannot be regarded as an effective purification method available in an industrial process.
On the other hand, in the method in which water is removed from the low boiling point components containing the ammonium carboxylate and water with the use of conventional distilling apparatus, the ammonium carboxylate is decomposed into a carboxylic acid and ammonia even at relatively low temperatures because its thermal stability is poor. The ammonia formed by the decomposition is distilled together with water off the top of the apparatus. When it is intended to recover the ammonia, an operation for further separating water from a fraction containing ammonia and water is needed. Thus, it is an nonefficient separating method and is not suitable to an industrial process. Therefore, there is a demand for the development of a simple economic technology for recovering the ammonium carboxylate from the low boiling point components containing the ammonium carboxylate and water.
As mentioned above, the carboxylic acid amide is produced by the dehydration reaction of the ammonium carboxylate. In the conventional method, side reactions occur and products of such side reactions (impurities) are concentrated and contained in the residue resulting from separation of the carboxylic acid amide from the reaction mixture. Although containing impurities much, this residue is composed mainly of the carboxylic acid amide. From the industrial point of view, it is desired to attain further recovery of the carboxylic acid amide from the above residue or attain effective utilization thereof, for example, recycling to the production stage.
Therefore, there is also a demand for the development of a process for producing a carboxylic acid amide, in which an effective utilization can be made of the residue resulting from recovery of the carboxylic acid amide from the carboxylic acid amide synthesizing reaction mixture and in which side reaction products that are detrimental to the quality of the carboxylic acid amide are not accumulated.